The present invention relates to a process for producing a refractory plate for a sliding nozzle apparatus used to control the flow of molten iron and molten steel in a steel making plant.
Refractory plates for a sliding nozzle apparatus to control the flow of molten metal have been indispensable refractory parts in steel making plants where a refining treatment in a ladle and continuous casting are now in general use. Such refractory plates for the sliding nozzle apparatus are required to have very high performance and to function to control the flow rate of the casting and withstand severe use conditions.
Refractory plates in the sliding nozzle apparatus experience strong thermal shock and wearing when coming into contact with high temperature molten steel. In addition to these physical actions, they are subject to mechanical erosion and chemical corrosion by the molten steel and slag. Therefore, refractory plates for the sliding nozzle are required to have high spalling resistance and high strength, as well as high corrosion resistance.
In order to fulfil such requirements with materials of the sliding plate having balanced properties, many efforts have been made. Recently, as a result of such efforts, alumina-carbon has come into use in the steel industry because of its stable durability.
Alumina-carbon-based refractory material for the sliding nozzle plates does not need to use pitch and tar which causes fumes harmful to health in use, and makes possible the production of a plate superior in durability compared with conventional pitch-impregnated ceramic bond plates. Generally, the refractory plates for the sliding nozzle are used without preheating, and rapidly heated from room temperature to 1500.degree. C. or more of the molten steel. This rapid heating subjects the refractory material to an extremely great thermal shock, and, simultaneously, to physical and chemical wearing when it is practically used for controlling the flow of molten steel. In the case of alumina-carbon-based refractory material, specified problems such as absorbing carbon into the molten steel therefrom and oxidizing of carbon in the refractory by the atmosphere are recognized, which makes it very important to select a proper carbon source for the refractory. For this reason, investigations are now being made to improve the materials constituting the alumina-carbon-based refractory plates for the sliding nozzle with the aforementioned problems in mind. For example, Japanese Patent Publication No. 58-020901 discloses the effect of fine carbon powder having a specific surface area greater than 50 m.sup.2 /g; Japanese Patent Publication No. 56-016112 discloses the effect of using a carbonaceous material in combination with sillimanite mineral material; Japanese Patent Application laid-open No. 56-096775 discloses the effect of adding ZrO.sub.2 -Al.sub.2 O.sub.3 -SiO.sub.2 -based raw material; Japanese Patent Application Laid-open No. 58-125660 describes the effect of Al.sub.2 O.sub.3 -ZrO.sub.2 -based material; Japanese Patent Application Laid-open No. 60-060973 describes the combined use of pitch and phenolic resin; and Japanese Patent Application Laid-open No. 60-003027 describes the combined use of novolak-type phenolic resin and resol-type phenolic resin.
Properties of the carbon to be incorporated into alumina-carbon-based plates are important. For example, according to Japanese Patent Publication No. 58-020901, carbon black is used as the major carbon source because it is insoluble in molten steel and highly resistant to oxidation by molten steel, which contribute to improve the durability of refractory plates. However, the amount of carbon black to be added should be limited to in the range of from 3 to 7 wt. % because an amount less than 3 wt. % leads to deficiency in corrosion resistance and an amount in excess of 7 wt. % causes a lack of spalling resistance. In order to overcome such disadvantage, improvements were proposed in Japanese Patent Application Laid-open Nos. 58-125660, 60-060973, and 60-003027. Although these proposed inventions are effective to a certain extent, they are not effective enough to eliminate damages caused by molten metal sticking on the sliding surface. A means to reduce such damage is to increase the amount of carbon in the composition. However, merely increasing the amount of graphite or amorphous carbon causes promoted dissolution of carbon in the molten metal, which in turn leads to damage to the edge of the nozzle hole and incomplete stopping of molten steel flowing. As a result, increasing carbon content does not contribute to lengthening the life of the plates. Further, improvements in thermal properties by changing the features of coke derived from a binder is also proposed in Japanese Patent Application Laid-open Nos. 60-060973 and 60-003027. However, these proposed methods are not very effective preventing the sticking of the molten steel.
The conventional alumina-carbon-based refractory have good resistance has chemical corrosion by molten slag, but has a disadvantage in that it is difficult to obtain a high compact and a high strength structure because of a lack of uniform dispersion and sintering ability of fine powder constituents due to no-sintering properties of carbon powder in the manufacturing process. Particularly, it is recognized that the matrix structure is easy to peel-off when removing the steel sticking in practice.